1. Field of the Invention
The present invention relates to a development device and an image forming apparatus such as a copier, a printer, a facsimile machine, a plotter, and a multi-function machine that includes the same.
2. Discussion of the Background
Electrophotographic image forming apparatuses such as copiers, printers, facsimile machines, plotters, multi-function machines, or the like typically include a development device and a transfer unit. The development device develops a latent image formed on a photoreceptor serving as a latent image carrier into a visible toner image. The transfer unit transfers the toner image from the photoreceptor onto a recording medium (e.g., transfer sheet) to form an image on the recording medium.
At present, the development device develops latent images into toner images using either one-component developer or two-component developer. The one-component developer consists of magnetic or non-magnetic toner. On the other hand, the two-component developer includes toner and carrier particles for carrying the toner. The development device develops the latent image formed on the latent image carrier with the developer including the toner and the carrier.
As for the development device using two-component developer, a configuration that includes a developing sleeve serving as a developer bearing member to bear the developer on a surface thereof and enwrap a magnetic-field generating member that generates multiple magnet poles is known. This development device includes a supply path from which the developer is conveyed to the developing sleeve in a direction parallel to an axis of the developing sleeve. Toner is consumed to develop the latent image on the photoreceptor and accordingly toner concentration (the content of the toner in the developer) of the developer after passing through the developing range decreases. If the developer supplied from the supply path to the developing sleeve that has passed the developing range formed between the photoreceptor and the developing sleeve is directly sent back to the supply path, the toner concentration of the developer decreases as the developer flows downstream in the supply path in a direction of conveyance of the developer (hereinafter “developer conveyance direction”).
Therefore, there are development devices that further include a recovery path through which the developer that reaches an extreme downstream of the supply path is transported to an upstream side of the supply path.
Published Unexamined Japanese Patent Application No. (hereinafter referred to as JP-A-) H05-333691 discloses a development device that includes supply path and a recovery path disposed in parallel to the developing sleeve. In this development device, developer is supplied from the supply path to the developing sleeve, and the developer supplied to the developing sleeve is conveyed to the recovery path after development, that is, after passing through the developing range. Specifically, the developer having lower toner concentration after passing through the developing range is not returned to the supply path but is instead conveyed to the recovery path and discharged from the sleeve. As a result, the toner concentration of the developer at a downstream end of the supply path relative to the direction of conveyance of the developer is not decreased.
In this example, a supply screw, serving as a supply conveyance member to convey the developer, formed of a rotation shaft with bladed spiral portion, is provided in the supply path, and the developing sleeve is disposed above the supply screw. The developer in the supply path is pumped up to the developing sleeve due to a magnetic force exerted by a magnetic field generating member provided in the developing sleeve.
However, when the supply screw is used for the supply conveyance member, the level of the developer in the supply path fluctuates following the shape of the spiral blade. Due to the fluctuation in the supply amount of the developer to the developing sleeve, unevenness of image density (screw pitch fluctuation) is generated.
In addition, in this development device, because the developer supplied from the supply path to the developing sleeve is not returned to the supply path, the amount of the developer decreases gradually as the developer flows downstream in the supply path, and the developer gradually increases as the developer flows downstream in the recovery path. Therefore, the dispersion of the amount of the developer is uneven in the development device. Consequently, when the amount of the developer supplied to the developing range fluctuates, even when the toner concentration of the developer is uniform, images of uneven image density are generated.
Accordingly, in order to prevent the decrease of the developer as the developer flows downstream, in this development device, rotation velocity of the supply screw and the recovery screw can be increased, so that a sufficiently larger amount of the developer is conveyed in the supply path and the recovery path. As a result, unevenness in the amount of the developer at the upstream end and the downstream end in the supply path is reduced. However, faster rotation velocity of the screws causes larger stress to the developer, resulting in deterioration in the developer. Further, the faster rotation velocity of the screws also causes an increase in torque, causing abrasion of mechanical components such as roller bearings. In other words, there is a limit to how much the rotation velocity of the screws can be increased, imposed by the need to prevent shortening the life of the development device and the developer. In addition, the cost is increased.
As another example, JP-H11-167261-A proposes a development device in which a supply path is provided above the developing sleeve, a wall separates the supply path from the development sleeve, and the developer is supplied from the supply path to the developing sleeve via a communication pathway provided above the wall separating the supply path and the developing sleeve. The developer from the supply path passes over the wall, dives, and then is supplied to the developing sleeve under its own weight to the surface of the developing sleeve 34a. 
In this example, while the developer passes over the wall and then dives to the surface of the developing sleeve, the unevenness of amount of the developer (screw pitch fluctuation) supplied to the developing sleeve can be prevented. However, because the developer is conveyed by only gravity, the developer may accumulate close to an upper end of the wall. If the developer accumulates, image failures such as image fade or white lines in image may occur.
In addition, in this example, the wall between the supply path and the communication pathway is higher the farther downstream because the level of the developer is higher on the upstream side and lower on the downstream side in the supply path. In this example, the unevenness in the supply amount of the developer to the developing sleeve can be ameliorated.
However, the level of the developer fluctuates with time in this development device, and when the excessive developer is conveyed to the developing sleeve, the amount of the developer conveyed to an area between the developer regulator and the developing sleeve is increased, and the excess pressure is exerted on the developer and therefore, the developer deteriorates and the life of the developer is decreased.
Accordingly, there is a need for a technology to better control the supply of developer to the developing sleeve.